1. Field of the Invention
The present invention is directed to a medical diagnostics system wherein a subject is irradiated with x-rays to produce an image, of the type having means for automatically controlling the exposure.
2. Description of the Prior Art
An x-ray diagnostics unit having an automatic exposure unit for measuring the dose rate and thereby controlling an adjustment element for the x-ray tube voltage is disclosed by German OS 21 35 205. In this known system, an x-ray radiator produces an x-ray shadowgraph on an x-ray film of an examination subject arranged between the x-ray film and the x-ray tube. The x-radiation that penetrates through the x-ray film is incident on a measuring chamber of an automatic exposure unit that ends the exposure after a defined radiation dose has been reached.
German OS 31 06 627 discloses an x-ray diagnostics system having a control circuit for controlling the exposure. In this x-ray diagnostics system, an image intensifier video chain receives the radiation that is emitted by a radiation transmitter and penetrates through an examination subject, this radiation being converted into a visible image of the examination subject. The image intensifier video chain includes an image intensifier with a video pick up tube coupled thereto by an optical coupling element, a video intensifier and a monitor.
The control circuit for controlling the exposure includes circuitry for forming a control voltage for the dose rate of the x-ray tube. A rated value generator and a circuit that contains an evaluation circuit and means for the blanking of parts of the video signal in order to form a dominant field are connected to the control circuit. An actual value generator is connected to the photo-cathode of the x-ray image intensifier and to the high-voltage generator thereof. The actual value generator supplies, the control circuit with an actual value signal that corresponds to the average image brightness. The actual value signal, the rated value signal and the output signal of the circuit are superimposed on one another in the control circuit as a correction value of the rated value.
An x-ray diagnostics unit having an x-ray image intensifier video chain is disclosed by German OS 32 25 061. A video camera is coupled to the x-ray image intensifier via optics having a base objective and a camera objective. A mirror that couples a part of the light stream onto a light detector, composed of a matrix of photo-sensors, lies in the optical beam path. The parallel outputs the photo-sensors are connected via switches to a summing amplifier of a test circuit, which includes means for setting the rated value. The output of the test circuit regulates a high-voltage generator of an x-ray radiator. Any desired part of the x-ray image can be selected by the switches as a measurement dominant, or a plurality of parts can also be interconnected. Regions of the measuring dominant can be differently weighted via variable resistors that follow the photo-sensors. An integration of their signals can ensue via amplifiers and capacitors that follow the photo sensors. A peak weighing of these signals is enabled when a respective diode is connected between the amplifiers and the capacitors.
In these known exposure controls, the control of the x-ray radiator ensues by regulating operation of the x-ray radiator to achieve a defined blackening of an x-ray film within a predetermined measuring dominant. The signal acquired by the radiation sensor within the measuring dominant, however, is not only disadvantageously influenced by the primary radiation, but also by the scattered radiation and, particularly if an x-ray image intensifier video chain is used as the radiation receiver, by the low frequency drop (LFD). The low frequency drop arises due to the rough contrast behavior of the x-ray image intensifier. The rough contrast behavior of the x-ray image intensifier is expressed by a defined drop of the modulation transfer function (MTF), even given extremely low spatial frequencies. This undesired effect is caused, for example, by reflections in the x-ray image intensifier and, given a transparency discontinuity in the examination subject, causes the corresponding brightness discontinuity at the output luminescent screen of the x-ray image intensifier to appear blurred or spread. Given the presence of direct radiation, i.e. extremely high radiation intensities next to the subject region of interest, a large part of the "bright" region thus extends into the normally exposed region, corresponding to a differential auxiliary exposure with information-free steady radiation. When, as is usual in practice, the measurement dominant of the automatic exposure unit lies in the region of interest, then the dose rate is regulated down to the rated value of the brightness due to the light level raised by the LFD component. By contrast to the absence of direct radiation, however, this means that less of a dose rate is applied and thus greater quantum noise also appears.